This invention relates to disk drives for computer systems. In particular, this invention relates to methods for detecting errors during write operations in disk drives, especially those caused by transient increases in flying heights.
Maximizing the reliability of the data in disk drives, both magnetic and optical, is a key objective of disk drive designers. Unfortunately, that objective often conflicts with the similarly important need for performance. That is, by increasing the reliability performance measures, other performance characteristics such as the data transfer rate, could suffer. For example, when data is written to the disk drive the success of the write operation, i.e., whether the media accepted the data, is unknown. One sure way to guarantee the data after each write operation is by re-reading the recorded data after each write operation. However, a technique that requires all data written to be read as well would severely degrade the performance of the drive. On the other hand, such verification would ensure high reliability of the data. This verification procedure is referred to as a xe2x80x9cwrite with verify.xe2x80x9d Write with verify takes a long time to perform. It requires that the data be written, then the disk is rotated one fill revolution, the data is read, and then verified.
U.S. Pat. No. 5,588,007, Ma describes a method for detecting transient write errors based on difficulty in reading pre-recorded information on the disk such as servo marks, ID marks and others. It also discusses that in the event of such difficulty, the drive will automatically read the data it just wrote. If errors were encountered during this write process, the drive will either re-write the data or report errors to the host system which will in most cases issue a rewrite operation. However, the pre-recorded information covers only about 20% of the disk space. Therefore, this technique cannot catch all hard errors caused by bad writes. This technique catches about 30% of non-recoverable data errors in one implementation.
In disk drives where (G)MR heads are used, an ID-less format further reduces the effectiveness of the transient error detection method described in U.S. Pat. No. 5,588,007. K. B. Klaassen, J.C.L. van Peppen,xe2x80x9cElectronic Abatement of Thermal Interference in (G)MR head Output Signalsxe2x80x9d, IEEE Transactions on Magnetics, Vol. 33, No.5, Sep. 1997 and U.S. Pat. No. 5,650,887 Dovek, et al. describe what is referred to as (G)MR (Giant Magneto Resistance) Technology which is commonly used in heads for hard disk drives. (G)MR heads have been used in rigid disk drives, where a (G)MR read transducer and an inductive write transducer are built into one slider. (G)MR technology is used to increase the density requirement. This increase in linear density possible with (G)MR Technology makes the xe2x80x9cphantom writexe2x80x9d problem more severe. Axe2x80x9cphantom writexe2x80x9d is a non-recoverable data error caused by temporary spacing loss during a write. This leads to a greater need for the type of automatic transient error detection method described in U.S. Pat. No. 5,588,007.
Similarly in optical recording systems, such as CDRW drives, the only way to verify that data was written correctly is to read it back, taking additional time.
It is an object of the present invention to improve transient error detection to make it more effective in higher density magnetic and optical disk drives.
In accordance with the present invention, the integrity of data written on a magnetic or optical disk is verified by writing data to a portion of the disk, immediately reading the data from the disk after it has been written, and, if the data read from the disk is substantially different from the data written to the disk, generating an error condition. xe2x80x9cImmediately,xe2x80x9d as used herein, means within one revolution of the disk.
The present invention recognizes the fact that in recording heads where (G)MR sensors are used there are two transducers, a (G)MR sensor for reading the data and an inductive sensor for writing the data. Furthermore, the two transducers are arranged such that they are in very close proximity of each other and usually line up in the recording track direction. Ideally, the (G)MR read sensor is located towards the trailing edge of the recording head, making it possible to read the data that was just written by the write sensor.
In the case of optical drives, a separate optical read path is provided that can be active simultaneously with writing. Immediately after writing, this optical read path allows verification of the written data. The read path includes appropriate optics and mechanics to place a read spot trailing the write spot by a small amount on the disk. This read spot can be attached to the optical pick up unit (OPU) fine actuator. The read spot can be generated by custom optics such as a hologram from the right laser diode, or it can be generated from a separate low power read laser diode. The reflected light from this read spot is directed to a detector that does not receive the reflected light from the right spot. Thus reading the data occurs immediately after writing the data.
The foregoing objects, features, and advantages of the invention will be better understood from the following more detailed description and appended claims.